When I was at school (UK) there was a German girl and a Swiss girl talking in German and one of them said “Scheiser” really loudly, the teacher turnes and glared at them and slowly they realised why and had to apologise to the teacher, as they hadn’t really thought that while he couldn’t understand, he still knew the swear words!

Wasn’t a big deal was just funny and everyone laughed (incl teacher), needless to say they were a bit more cautious after that.